<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
      <title>a nav</title>
          <style type="text/css">
*{margin:0;padding:0;font-style:normal;font-family:'微软雅黑';}
body{text-align:center;font-size:14px;}          
#content{margin:0 auto;width:600px;}
#content #nav{background:#006400;height:32px;margin-top:60px;}
#content #nav ul{list-style:none;}
#content #nav ul li{float:left;width:100px;line-height:32px;position:relative;}
#nav div{width:100px;position:absolute;left:0px;padding-bottom:0px;background:#006400;float:left;height:0;overflow:hidden;}
#content #nav li .a{text-decoration:none;color:#00CD00;line-height:32px;display:block;border-right:1px solid #009800;}
#nav div a{text-decoration:none;color:#00CD00;line-height:26px;display:block;}
#nav div a:hover{background:#005400;}
</style>

</head>
<body>



<div id="content">
<div id="nav">
<ul id="supnav">
<li><a href="#" class="a">好好学习</a>
<div>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
</div>
</li>
<li><a href="#" class="a">好好学习</a>
<div>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
</div>
</li>
<li><a href="#" class="a">好好学习</a>
<div>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
</div>
</li>
<li><a href="#" class="a">好好学习</a>
<div>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
</div>
</li>
<li><a href="#" class="a">好好学习</a>
<div>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
</div>
</li>
<li><a href="#" class="a">好好学习</a>
<div>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
<a href="#">好好学习</a>
</div>
</li>
</ul>
</div>
</div>
<script type="text/javascript">
var supnav=document.getElementById("supnav");
var nav=document.getElementById("nav");
var btns=document.getElementsByTagName("li");
var subnavs=nav.getElementsByTagName("div");
var paddingbottom=20;
var defaultHeight=0;
function drop(obj,ivalue){
var a=obj.offsetHeight;
var speed=(ivalue-obj.offsetHeight)/8;
a+=Math.floor(speed);
obj.style.height=a+"px";                                                         
} 
window.onload=function(){
for(var i=0;i<btns.length;i++){
btns[i].index=i;
btns[i].onmouseover=function(){
var osubnav=subnavs[this.index];
var sublinks=osubnav.getElementsByTagName("a");
if(osubnav.firstChild.tagName==undefined){
var itarheight=parseInt(osubnav.childNodes[1].offsetHeight)*sublinks.length+paddingbottom;
}
else{
var itarheight=parseInt(osubnav.firstChild.offsetHeight)*sublinks.length+paddingbottom;}
clearInterval(this.itimer);
this.itimer=setInterval(function(){drop(osubnav,itarheight);},30);
}
btns[i].onmouseout=function(){
var osubnav=subnavs[this.index];
clearInterval(this.itimer);
this.itimer=setInterval(function(){drop(osubnav,defaultHeight);},30);
}
}
}
</script>
</body>
</html>